It is known to make resonant filters from acoustic resonators of the SAW (surface acoustic waves) or QCR (quartz crystal resonator) type. The main drawback of these resonators is their relatively large size. There are also miniaturized resonators of the FBAR (film bulk acoustic resonators) or BAW (bulk acoustic wave resonators) type smaller than the SAW or QCR resonators. However, such resonators are still bulky, in particular due to their support membranes, and are also difficult to manufacture.
To resolve this bulk problem, it is possible to produce RF filters from resonators of the MEMS (micro-electro-mechanical systems) type having a reduced bulk and low consumption. In addition, these resonators make it possible to obtain high resonance frequencies and very high quality factors, and are easy to produce. Such resonators can also be integrated with the electronic processing part of the filter on a same chip.
It is known to mechanically couple several MEMS resonators in order to implement signal processing functions able to be carried out by transistor circuits, but with more power and better linearity. These coupled resonators make it possible to obtain different types of filtering on a broader bandwidth than that obtained by an individual resonator. However, the responses obtained by coupled resonators depend greatly on individual parameters of the resonators used and their operating states. Depending on the aim of the application, the number of resonators used varies and the coupling elements are modified.
The mechanical coupling of MEMS resonators has several drawbacks, and in particular the dispersion of the parameters of the resonators due to the hazards of the mass production of resonators, leading to poor knowledge of the global system, as well as the difficulty of integrating it due to its larger size. Although using NEMS (nano-electro-mechanical systems) reduces the size of the filter, it demonstrates still more problems due to the complexity of modeling couplings of a large number of resonators on the nanoscopic scale, revealing unexpected responses from the system at higher resonance modes. Furthermore, the NEMS provide output signals with lower amplitudes than those obtained by MEMS resonators, which is a major problem in the presence of electronic noise, transmissions, etc.
Furthermore, such resonator couplings have drawbacks related to the non-linearity and discontinuities in the phase of the response. Lastly, one problem that is very present in controlling such systems is the variation of their parameters with their operating states and the ambient environment, which generally reduces the performance of the associated electronic circuit.
The document J. R. CLARK et al., “Parallel-Coupled Square-Resonator Micromechanical Filter Arrays”, Proceedings of the 2006 IEEE International Frequency Control Symposium, pages 485 to 490, describes the production of a filter from mechanically coupled MEMS resonators. Resonators connected in a series make it possible to decrease the series resistance of the system representing the damping. The MEMS resonators are also connected in parallel by coupling beams having a lower stiffness to increase the system's bandwidth.
In order to meet more demanding filtering characteristics or perform more sophisticated functions, it may be necessary to couple a larger number of resonators (hundreds or thousands of resonators). This greatly complicates the manufacture of such a system and the parameters of the resonators are then more dispersed and uncertain. Subsequently, the necessary coupling elements may be poorly calculated and the resonant frequencies as well as the bandwidths obtained are then poorly defined and the responses obtained end up being erroneous in relation to the desired responses. Other problems also arise, such as the noise caused by heat fluctuations, electronic noise, and gaps in the power management. Furthermore, the gains in the responses obtained are low and the global phase of the response obtained cannot be exploited due to its significant distortion. Added to these problems is the environment factor, which modifies the quality factors of the resonators and makes the electronic processing and control part much lower performing in relation to the case of nominal operation.
The document “Design considerations for an acoustic MEMS filter” by S.-H. Shen, Microsystem Technologies 10, pages 585-591, 2004, describes a device using MEMS resonators to process acoustic signals. This device includes an electronic processing circuit making it possible to couple the outlets of the circuits detecting the resonance of different MEMS resonators in order to obtain a desired bandwidth.
Such a device does, however, involve the use of imposing and bulky processing electronics to make the switches necessary for coupling the outlets of the detection circuits of the resonators. Moreover, the dispersion in the obtained results, due to the uncertainties related to the method for making the resonators, is not taken into account and the global response obtained may be distant from that which should theoretically be obtained. Lastly, in this device, detection circuits perform the vibration-electrical signal conversion for each resonator. These detection circuits comprise complex processing electronics and are therefore costly to produce given the number of resonators used.